Anthrax toxin is being used as a model system for the insertion of water-soluble protein toxins into membranes. Anthrax protective antigen (PA) requires proteolytic activation and acidic pH for insertion into the endosomal membrane of target cells. Using both conventional EM and the STEM, the active form of PA in solution was shown to be heptameric rings. A second component of the toxin, lethal factor (LF), appears to interact with the activated PA rings. Current studies are exploring whether the binding of LF is only on the outside of the ring structures or is also within the pore. Recent work has been looking at rearrangements in the fine structural details of PA as a function of pH.